1. Field of the Invention
The present invention relates to an electronic control apparatus used in an electric power steering system for providing an assist force to a steering system of a vehicle by means of the rotational force of an electric motor.
2. Description of the Related Art
In the past, there has been known an electronic control apparatus in which a semiconductor switching element (FET), being a power device, is mounted on a metal substrate, and at the same time, a connecting member for electrically connecting between the metal substrate and parts outside the metal substrate is mounted on the metal substrate.
For example, an electronic control apparatus described in a first patent document (Japanese patent No. 3644835) includes a power board on which a bridge circuit comprising semiconductor switching elements are mounted for switching a current supplied to an electric motor, a housing with conductive plates, etc., insert molded into an insulating resin and having high current parts mounted thereon, a control board having low current parts such as a microcomputer, etc., mounted thereon, a connecting member for electrically connecting the power board, the housing and the control board to one another, and a heat sink being in intimate contact with the power board, and a casing press molded from a metal plate so as to cover the power board, the housing and the control board and mounted on the heat sink.
In addition, an electronic control apparatus described in a second patent document (Japanese patent application laid-open No. 2006-49618) includes a metal substrate, a housing having a substantially box-shaped outer shape, a control board formed of an insulated printed-circuit board, a heat sink made of aluminum, fixedly secured to an aperture in the housing at one end thereof, and having a metal substrate, and a cover fixedly secured to an aperture in the housing at the other end thereof so as to cover the control board, wherein individual rubber rings are inserted into grooves formed in the housing and the heat sink, respectively, so as to provide sealings between the aperture in the housing and the heat sink and between the aperture in the housing and the cover.
In the electronic control apparatus described in the above-mentioned first patent document, the power board on which the semiconductor switching elements are mounted is required.
Although the connecting member is fixedly secured to the power board so as not to float at the time of soldering, an impact force generated when the connecting member is fixed onto the power board is transmitted to component parts such as the semiconductor elements onto the power board before soldering thereof, so there occur positional shifts of such component parts.
As a result, there is the following problem. That is, the number of parts required increases, thus making the size of the electronic control apparatus larger and the production cost thereof higher, and the reliability of soldered joints of the parts mounted on the power board is reduced.
Also, there is another problem. The power board, the housing and the control board are covered with the casing which has been press molded from the metal plate, so it is difficult to seal the main body of the apparatus, and in order to mount the apparatus on the engine room of a vehicle, a device of a sealed structure is separately required so as to prevent water from being infiltrated into the apparatus main body due to the pour water thereof, so separate production processes have to be used depending upon whether such a sealing device is required or not.
Further, in the electronic control apparatus described in the second patent document, there is the following problem. That is, to prevent the infiltration of water into the apparatus main body, the rubber rings for sealing are needed, and the cost for production becomes high.
In addition, there is also another problem. When the rubber rings with small rigidity are inserted into the groove formed in the heat sink and the groove formed in the housing, respectively, the time and effort are required, thus resulting in poor assemblability.